36 seamanship secretsAPPLYING VARIATION TO FIND YOUR MAGNETIC
TRACKLINE
You can read true directions from the outermost ring of the compass rose and
then convert these to magnetic directions, or you can use the inner ring to
plot magnetic directions directly. Many small-boat navigators prefer the latter,
but I fi nd the inner ring diffi cult to read because the degree hash marks are
densely packed and oft en labeled only at 30-degree intervals (in contrast with
10-degree intervals for the outer ring). It’s a simple process to fi nd magnetic
directions from true. You begin by plotting the true direction, then add a
westerly variation or subtract an easterly one.
Th e only tricky part is remembering when to add and when to subtract.
Some navigators use the mnemonic “East is least (–); west is best (+).” When
converting from true (indicated as T) to magnetic (indicated as M), you subtract
an easterly variation and add a westerly one.
The true course to Shelter Cove is
135 degrees true. If the local variation is
17 degrees east, then your magnetic course
would be: 135 degrees true – 17^ degrees
east = 118 degrees magnetic.Check Your Steering Compass in Th ree Easy Steps
It is utterly reckless to rely upon an
uncorrected compass.
—Malcolm Pearson, Reed’s
Skipper’s HandbookThe small magnets on the bottom of
a compass card love to interact with
metals and electrical currents on your
boat. This can pull the compass card
away from its proper reading by several
degrees. The resultant error is called
deviation. Unlike variation, deviation is
peculiar to each boat and changes with
your heading.
I remember being sent out on
a Coast Guard rescue case offshore
on an open 40-foot patrol boat. OurInner Ring versus Outer
I rarely use the inner (magnetic
variation) ring on charts, and here’s
why.
First, on many charts, it’s on a
smaller scale than the outer ring, and
shows degrees in two-degree versus
one-degree increments. That means
more chance for a misread, especially
if the rose shows the compass points
just inside the magnetic ring. More
confusion!
Second, you are dependent on a
compass rose that may lie far from
your position. Trying to walk parallel
rulers or a protractor across a large
space leads to errors when they slip.
Third, on offshore charts, cartog-
raphers don’t include the magnetic
ring on roses; only true directions
are shown. This is because variation
changes so quickly offshore (smaller-
scale charts) that your chart would
be cluttered with compass roses.
That’s why magnetic variation on
offshore charts is shown with diagonal
isogonic lines.